A dynamometer is a diagnostic test instrument that measures power and torque as a function of speed, either in terms of revolutions per minute (rpm) at the engine or clutch, or in terms of ground speed. The power output accelerates an inertia flywheel, typically a steel drum with a known mass. Sensors from the engine and flywheel are coupled to a computer to compute the output power and torque. Data on engine speed, flywheel speed and environmental inputs are collected. The data, together with the known moment of inertia of the flywheel, is used in a known engineering formula for the calculation of power plant performance including horsepower and torque. The typical computed measurement output of a dynamometer is a pair of graphs showing output horsepower versus engine or clutch rpm (or ground speed) and output torque versus engine or clutch rpm (or ground speed).
There are two types of dynamometers: an engine dynamometer and a chassis dynamometer. An engine dynamometer measures the output of the engine alone, while a chassis dynamometer measures output at the driven wheels.
To use an engine dynamometer, the engine is removed from the vehicle, mounted on a test bench, and coupled to the dynamometer. Some vehicle designs permit an in-chassis dynamometer test by providing for partial disassembly in order to access the engine output. Engine dynamometers present several problems. For one, there is the difficulty of removing an engine from a vehicle, and then reinstalling the engine in the vehicle after the test. Even for an in-chassis test, there is the difficulty of partial disassembly and reassembly. Furthermore, engine characteristics alone do not represent a complete picture of vehicle performance. Problems with drive chains, transfer cases and the like, are not evident from an engine dynamometer run. Other components in the power train, such as the clutch, transmission, and wheels greatly affect the ultimate characteristics of the vehicle. In the case of a snowmobile, an engine dynamometer test would not uncover any problems with the drive lines, chains, clutch transfer case or track. In contrast to an engine dynamometer, a chassis dynamometer measures the output at the driven wheels of a motor vehicle. A chassis dynamometer takes into account the effect of the drive train and engine together. Chassis dynamometers for automobiles and motorcycles are known. In a chassis dynamometer, the drive wheel or wheels of the vehicle are placed on top of the dynamometer so that each driven tire contacts an inertia flywheel which is directly accelerated in a dynamometer run. The vehicle is secured by its non-drive wheel or wheels. Some dynamometers use a pair of wheels to cradle the driven wheel(s) of the test vehicle. A chassis dynamometer for snowmobiles would present special problems because the track of a snowmobile does not easily drive the smooth surface of the flywheel.